Secondary batteries which are highly applicable to various products and exhibit superior electrical properties such as high energy density, etc. are commonly used not only in portable devices but also in electric vehicles (EVs) or hybrid vehicles (HVs) driven by electrical power sources. The secondary battery is drawing attentions as a new energy source for enhancing environment friendliness and energy efficiency in that the use of fossil fuels can be reduced greatly and no byproduct is generated during energy consumption.
The secondary battery can be classified into various kinds depending on type, structure, etc., including a can-type battery in which its internal components are accommodated in a hard metal case. Recently, demand on a prismatic battery and a pouch-type battery with small thicknesses is increasing as the mobile devices are becoming smaller in size. In particular, the pouch-type secondary battery which is advantageous in terms of easy shape changing, low manufacturing cost and light weight is gaining a lot of attentions. Also, the pouch-type secondary battery is being developed and commercialized as a power source for electric vehicles or hybrid electric vehicles requiring high output and large capacity.
The pouch-type secondary battery includes an electrode assembly, an electrode tab extending from the electrode assembly, an electrode lead soldered to the electrode tab and a pouch housing consisting of a polymer resin and an aluminum laminate sheet which accommodates the electrode assembly. When the temperature inside the pouch-type secondary battery is increased due to overcharging exceeding permitted current or voltage, internal short circuits, etc., the internal pressure is increased due to evaporation of an electrolyte, etc., leading to swelling of the pouch housing. When this swelling phenomenon occurs, a local short circuit can occur as the battery is deformed. In extreme situations, the battery may catch fire or explode.
Therefore, one of main research topics on the pouch-type secondary battery is to improve safety. In this regard, methods for venting a gas generated inside the pouch housing to resolve the swelling phenomenon caused by increased internal pressure have been proposed. As an example, Korean Patent Application Publication No. 2009-0060497 discloses a pouch-type secondary battery equipped with a safety vent prepared by making fine holes on a metal plate used as an electrode tab such that a gas generated inside the battery can be expectably and easily vented under a battery abuse situation.
Although such gas venting is effective in decreasing the internal pressure of the battery, it is limited in resolving the basic cause of internal pressure increase such as overcharge, etc. That is to say, even after the gas is vented, overcharging from outside is continued because the electrode assembly and the electrode tab remain connected.
For cylindrical and prismatic secondary batteries, an overcharge safety device such as a CID (current-interrupting device) can be used because they employ rigid packaging materials. When the internal pressure of the battery is increased due to a gas generated by overcharging, the CID is operated and, at the same time, the electrode assembly is separated from the electrode tab, thereby interrupting current. For the pouch-type secondary battery, however, it is difficult to use the CID because easily shapeable packaging materials are used. Therefore, development of a method for fundamentally resolving the problems caused by overcharging and greatly improving safety by interrupting current when a swelling phenomenon occurs in a pouch-type secondary battery due to overcharge, etc. is necessary.